Memory is one type of integrated circuitry, and is used in systems for storing data. Memory is usually fabricated in one or more arrays of individual memory cells. The memory cells are configured to retain or store information in at least two different selectable states. In a binary system, the states are considered as either a “0” or a “1”. In other systems, at least some individual memory cells may be configured to store more than two levels or states of information.
Integrated circuit fabrication continues to strive to produce smaller and denser integrated circuits. Accordingly, there has been substantial interest in memory cells that can be utilized in structures having programmable material between a pair of electrodes; where the programmable material has two or more selectable resistive states to enable storing of information. Examples of such memory cells are phase change memory (PCM) cells.
PCM cells may comprise selectable memory states corresponding to a high resistance state (HRS) and a low resistance state (LRS). Traditional PCM cells have a phase change material (for instance, a chalcogenide; such as a so-called GST material comprising a mixture of germanium, antimony and tellurium) as the programmable material. In operation, a portion of the phase change material is altered in phase in the HRS state relative to the LRS state. For instance, the portion may be amorphous in one of the states, and crystalline in the other. Accordingly, the PCM cells may be transitioned between the HRS and LRS states by melting a region of phase change material, and then either crystallizing the material or amorphizing the material.
Problems may be encountered during the transitioning between the HRS and LRS states. For instance, the melting of the phase change material may induce segregation within such material, which can eventually degrade operation of the memory cells. Also, the temperature utilized for melting of the phase change material may lead to degradation of memory cells and/or associated circuitry over time. Further, the power consumption associated with achieving the melting temperature of the phase change material may problematically reduce battery life of portable electronic devices.
Efforts have been made to develop materials analogous to those utilized in PCM cells, but which transition between the HRS and LRS states through other mechanisms besides melting of the programmable material. For instance, solid-state memory has been developed which comprises so-called chalcogenide superlattices. The chalcogenide superlattices are comprised by multiple stacked layers of different chalcogenides (see, for example, US patent publication numbers 2011/0207284 and 2011/0315942). Such memory may transit from one memory state to another through changes in lattice structures and/or bonding, and thus may transition at lower temperatures than the melting temperatures associated with traditional PCM.
Problems may be encountered in attempting to form and utilize memory cells comprising stacked chalcogenides.
It is desired to develop improved memory cells which may have benefits analogous to those of stacked chalcogenides, but which may reduce, overcome, or entirely avoid the difficulties encountered with chalcogenide superlattices.